Piezoelectric thin films for bulk acoustic wave resonator applications

Bandpass filters for microwave frequencies realized with thin film bulk acoustic wave resonators (FBAR) are a promising alternative to current dielectric or surface acoustic wave filters for use in mobile telecommunication applications. With equivalent performance, FBAR filters are significantly smaller than dielectric filters and allow for a larger power operation than SAW filters. In addition, FBARs offer the possibility of on-chip integration, which will result in substantial volume and cost reduction. The first passive FBAR devices are now appearing on the market. They mainly cover needs in miniaturized RF-filters for the new bands around 2 GHz. A FBAR is essentially a thin piezoelectric plate sandwiched between two electrodes and acoustically isolated from the environment for energy trapping purposes. If the isolation is effectuated by an acoustic Bragg reflector, one speaks of solidly mounted resonators (SMR). Piezoelectric aluminum nitride (AlN) thin films are predominantly used in the emerging FBAR technology because AlN exhibits a sufficient electromechanical coupling coefficient kt2 , low acoustic losses at microwave frequencies, a low temperature coefficient of frequency, and its chemical composition is compatible with CMOS requirements. This thesis has two research directions. In the first part, FBAR structures based on AlN thin films were investigated for applications at X-band frequencies (7.2-8.5 GHz), i.e. operating at much higher frequencies than the ones used for present products. The goal was to identify property limitations related to such high frequencies, and to demonstrate to industry high performing SMR filters at 8 GHz. In the second part, a new material for FBAR devices was studied. The motivation is that AlN allows for a restricted filter bandwidth only, limited by its coupling factor of maximal 7%. Monocrystalline KNbO3 appears as an ideal alternative with its high coupling factor kt2 of 47%, and relatively large sound velocity of 8125m/s for longitudinal waves along the [101] direction. Piezoelectricity of KNbO3 films grown on electrodes has never been characterized. Single crystal results indicate that the optimal film texture would be (101). In this thesis, the growth of KNbO3 films on Pt electrodes was studied with the goal to achieve this texture uniformly, and to characterize piezoelectric properties. X-band FBAR's were first studied with numerical simulations based on a one-dimensional theory of the thickness-extensional bulk acoustic wave (BAW). Thickness, acoustic properties and electrical conductivity of the electrodes have a large impact on the resonator characteristics. There are conflicting requirements with respect to optimum acoustic and electrical properties of the electrode materials. An optimum thickness was calculated for 8GHz FBARs that use Pt bottom and Al top electrodes. The characteristics of ladder filters have been calculated based on the impedances of single resonators. The adjustable filter parameters, i.e. the areas of series and shunt resonators, frequency de-tuning between series and parallel resonators, and number of π-sections were screened for a process window offering maximum filter bandwidth with lowest ripple and low insertion loss for a given out-of-band rejection. An important result of the numerical simulations was that the bandwidth of ladder filters can be doubled by de-tuning the series and parallel resonators by more (1.3 times) than the difference of resonance and anti-resonance frequency. This also leads to a flatter passband while keeping the ripples below ±0.2dB. Solidly mounted resonators and filters were fabricated using an acoustic multilayer reflector consisting of AlN and SiO2 λ/4 layers. All films were sputter deposited in a high vacuum sputter cluster system with 4 process chambers. The films were patterned using standard photolithography and dry etching processes. The SMR exhibited a strong and spurious-free resonance at 8GHz with a high quality factor of 360 and electromechanical coupling coefficient of 6.0%. The temperature coefficient of frequency was -18ppm/K, and the voltage coefficient of frequency was -72ppm/V. Passband ladder filters with T- and π-topology consisting of 3 to 14 SMR were successfully demonstrated with a center frequency of 8GHz. These filters were optimized for maximum bandwidth and exhibited an insertion loss of 5.5dB, a rejection of 32dB, a 0.2dB bandwidth of 99MHz (1.3%), and a 3dB bandwidth of 224MHz (2.9%). There was good correspondence between measured and simulated filter and resonator characteristics. For perfect agreement, parasitic elements needed to be taken into account. These were a series resistance of 5Ω and a parallel conductance of 2mS in case of single resonators. The series resistance can be explained with resistive losses in the electrodes, whereas the parallel conduction was due to conduction along the surface. For π-filters, an additional series inductance of 100pH was needed to obtain a satisfactory fit. This inductance increased the out-of band rejection and insertion loss. Besides the group delay variation, all industrial specifications were met. KNbO3 was in-situ sputter deposited at 500 to 600°C using a rf magnetron source. A dedicated sputter chamber with load-lock and oxygen resistant substrate heater was built for this purpose. The high volatility of potassium oxide requires a potassium enrichment of the target. Targets with several excess concentrations (in the form of K2CO3) were studied. Stoichiometric KNbO3 films were obtained with targets containing 25 and 40% excess K. Zero and 10% excess yielded K deficient films, whereas 100% and 200% excess K led to highly unstable targets with K accumulation on the target surface, resulting in K rich second phases. The potassium-to-niobium ratio in the films depends strongly on sputter pressure and substrate temperature. Dense films, nucleated with cubic {100} texture, were obtained on platinized silicon substrates with a 10nm thick IrO2 seed layer at substrate temperatures of 520°C. At lower temperatures the films were amorphous, and at higher temperatures the films were composed of individual and facetted KNbO3 grains. The cubic high-temperature {100} texture results in a mixed (101)/(010) texture in the orthorhombic room temperature phase. The measured relative permitivity of 420 indicates that both orientations are equally present. Micro-Raman confirms the orthorhombic line splitting. Piezoelectrical and ferroelectrical activity were verified by means of a piezoelectric sensitive atomic force microscope. A very large piezoelectric activity was observed on some of the grains, and the polarization could be switched on most of the grains. However, the average d33,f = e33/c33, as measured by means of laser interferometry, showed a modest value of 24pm/V. The effective coupling factor is derived as kt2=2.8%, which is small relative to the theoretical value of 47%. The high dielectric constant and the absence of piezoelectric activity along the [010] direction are responsible for the reduction of the kt2 factor. Film roughness, complexity of deposition process and open poling issue make KNbO3 integration into BAW devices a difficult task.

Pulsed laser deposition of (SrBa)Nb2O6 thin films and their properties

Anna Infortuna

Strontium barium niobate (SrxBa1-xNb2O6, shortly SBN) is a solid solution system with tetragonal tungsten bronze crystal structure. It exhibits a ferroelectric phase with only one polar axis and a transition temperature depending on the Sr/Ba ratio. This thesis studies the growth of SBN thin films, in order to obtain a ferroelectric thin film model system for the study of 180° domain switching close to the ferroelectric phase transition. SBN has been chosen not only because uniaxial, but because it is possible to tune the temperature of phase transition varying the Sr/Ba ratio. It is thus possible to study electric properties of the phase transition close to room temperature, where conduction phenomena related to defects are normally lower. Thin films of SBN were grown by Pulsed Laser Deposition in a vacuum system constructed during this thesis work. For the formulation of a model it is desirable to have a system as close as possible to a single crystal. Therefore deposition parameters have been optimized to grow SBN thin films with the polar axis oriented perpendicular to the film plane. The films are integrated in parallel plate capacitor structures, in order to measure dielectric properties. The stability of the ferroelectric phase is found to be sensitive to impurities; contaminants with concentration below 1% induce a lowering of the transition temperature of about 100°C. Strain as well has influence on the phase transition; in the case of substrate with thermal expansion coefficients different from the SBN ones, the SBN film is under stress. A 0.6% tensile strain induces a lowering of the transition temperature of about 100°C, this is the case of silicon single crystal substrate. Grown on strontium titanate single crystals (STO), whose thermo-mechanical properties are close to the one of SBN, the films exhibit a phase transition temperature compatible with the one measured on single crystals. The obtained films suffer from leakage that is reduced by thermal treatment in oxygen rich atmosphere. The oxygen vacancies, created during the deposition process, are considered to be responsible of leakage. Vacancies recovery upon annealing does not eliminate the problem, and measurement of polarization at room temperature is difficult. However, piezoelectric measurements performed on individual grains, with atomic force microscope, prove that these have ferroelectric properties; by these experiment has been proved that it is possible to switch the polarization. Films with a disordered structure of grains are much less leaky than the ones with columnar grains spanning the whole cross section. These observations lead to the conclusion that conduction goes trough the grain boundaries. On STO single crystals, SBN grows epitaxial in Volmer-Weber mode. With a suitable preparation of the substrate surface, it is possible to induce the growth of films with different orientations of the polar axis: in the film plane, perpendicular to it or inclined of 45°. A model is proposed, explaining the epitaxial relation with the substrate on the basis of the perovskite kernel contained in the unit cell of the tetragonal tungsten bronze structure. The film grows from the nucleation of the perovskite structure that rules the orientation of the film; the high temperature at which the substrate is kept during the deposition, assure the necessary mobility for the arriving atoms to organize in the SBN structure around these nuclei. The study of literature data allows to state that such a model is valid not only for substrates with perovskite structure, but can be extended to substrates with a cubic crystal structure, like magnesium oxide.

EPFL2005

Theoretical and Experimental Study of Piezoelectric Modulated AlN Thin Films for Shear Mode BAW Resonators

Evgeny Milyutin

The use of acoustic resonators for sensors application has opened a new branch in research and applications of piezoelectric materials and devices. The first generation of such sensors is constituted by the quartz crystal microbalance (QCM) based on AT-cut mono-crystalline quartz. High sensitivities of gravimetric sensing in both air and liquids were demonstrated. Since the 1980s, when the first QCM-based sensor was demonstrated for the detection of silver in a liquid solution, many other application in chemical, bio-medical and environmental sensing were realized using the same concept. QCM's exhibit a very good thermal stability. The AT-cut quartz plate leads to the excitation of shear waves when used in parallel capacitor geometry. This is important for achieving high quality factors in the immersed operation of the sensor. A second generation of gravimetric sensors is based on surface acoustic wave (SAW) structures, working for instance with Love waves in a SiO2 layer on top of a LiTaO3 single crystal. SAW devices are mainly used as RF filters in television and mobile phones. SAW sensors are a side product of the much larger telecommunication market. The evolution of thin film and MEMS technology has lead to a third generation of gravimetric sensors that is based on bulk acoustic wave (BAW) resonances in piezoelectric thin films. Again, such sensors are a side product from the large telecommunication market where such resonators are used for RF filters. With every generation, the oscillation frequency increased. While QCM's operate typically at 5 MHz, the SAW resonators work typically at a few 100 MHz, and the thin film BAW resonators (TFBAR) operate typically around 2 GHz. The increase of the frequency goes together with an increase in sensitivity, and a decrease of the thickness and mass range that can be measured. The TFBARs are in some sense miniaturized analogs of QCM's operating at much higher frequencies. They are very promising as they reach higher sensitivities. This attracted the attention of researchers, and experimental evidence of the potential of TFBARs as sensors was delivered. In addition to the higher sensitivity, the miniaturization allows for using arrays of sensors with different immobilization layers as needed for drug screening. However, the application of the same TFBARs as used in telecommunications would not allow a good performance in immersed operation. It would only be good for operation in air. The principal characteristic of TFBARs used in mobile phones is the value of the electromechanical coupling and not the resonance mode at which this coupling is achieved. But for the in-liquid operated sensors, it's rather the opposite – the mode of the resonance should be such that the surface of the resonator, which tis in contact with liquid, should move parallel to the surface (in-plane motion, or transverse motion) to minimize emission of acoustic waves into the liquid. The coupling coefficient is of secondary importance in this case. Therefore, the development of shear mode TFBARs became an interesting task that was challenged by several research groups. One of the most successful solutions is the use of c-axis inclined AlN thin films. Inclination of c-axis in a parallel plate capacitor structure enables the coupling of the electric field to the shear strain, which enables the excitation of a shear mode. Such devices were successfully applied for selective sensing of organic species, such as DNA molecules, suspended in a liquid. Even if the process of deposition of c-axis inclined AlN films doesn't require hardware modification, the quality of the process is still far from the one for deposition of (0001)AlN films used in telecommunications. So the goal of this thesis was to find a solution for the shear mode TFBAR's based on (0001)AlN films. In (0001)AlN films, the shear strain cannot be induced by the electric field produced in a parallel plate geometry as the coefficient e35 and e34 of the piezoelectric tensor are zero. But there are e15 and e24 coefficients that are not zero, meaning that in-plane electric field can be used to excite the shear waves. In the frame of this work, this concept was studied theoretically and experimentally. The in-plane electric field was generated via inter-digitated electrodes (IDE). A first device type was realized in solidly mounted resonator (SMR) design and based on uniform (0001)- oriented AlN thin films. The anti-phase of the electric field in adjacent half-periods of the IDE resulted in an anti-phase movement of the corresponding regions of the film. Finite element modeling and boundary element modeling (FEM-BEM) were carried out to clarify the kind of vibrations present in the device. A kind of shear/lingitudinal mode with elliptic motions was obtained. A device was fabricated and tested both in liquid and air. The resonance of the device was observed in the expected frequency range (1.86 GHz) and high quality factors under operations in air (Q=870) and silicon oil (Q=270) were obtained. The drop of the quality factor was explained by the up and down motion of the regions of the film located directly under the IDE electrodes. Such a motion is due to anti-phase motion of different regions of the film as mentioned above. To prevent this effect, a second device type was studied. It is again based on the use of (0001)AlN thin films, but with modulated piezoelectric properties. Having different piezoelectric properties in the regions corresponding to adjacent half-periods of IDE, breaks the mirror symmetry of the device and allows for coupling the electric field to a pure shear mode. Analytical and numerical models explaining such a device were established and evaluated. The optimal situation is found when perfect Al-polar and N-polar regions of AlN are combined. This maximizes the coupling coefficient k that is derived as being proportional to the difference of e15 coefficients. Finding a way to fabricate the AlN thin film with different piezoelectric properties was thus the next objective. Growth features to decrease the piezoelectric effect were first studied. Providing rougher regions on the otherwise smooth substrate allowed to modify locally the quality of AlN thin films. Device based on such films were fabricated and characterized. The resonance of a pure shear mode was found at the expected frequency (roughly 2GHz) when the piezoelectric effect was modulated. Devices without this modulation failed to show the resonance at the exact frequency, exactly as the theory predicted. We managed to reduce two times the d33 coefficient of the film by inducing a increased roughness to the substrate – from 5.0 pm/V, corresponding to 1.5 degree rocking curve, down to 2.4pm/V, corresponding to 7 degree of rocking curve. The final step of the thesis was the process development for the simultaneous growth of Al-polar and N-polar regions within sputter deposited (0001)AlN, in order to achieve the maximal possible "piezomodulation" effect. As the sputter deposition yielded only N-polar films, we included Al-polar films from another source as seed layers. High temperature epitaxial growth methods of GaN and AlN on Si(111) and Si(100) lead to Ga- and Al-polarities. On such films, the sputter deposited AlN copies polarity from the growth substrate. The selective polarity was then obtained by preventing the expitaxy locally through a patterned oxide layer. Wet etching tests together with PFM measurements were performed to prove the dual polarity in the sputter deposited film. Finally, the integration of this process into the process flow for device fabrication was investigated.

EPFL2011

Conductivity, dielectric and piezoelectric properties of SrBi4Ti4O15

Strontium Bismuth Titanate is a very promising material for high temperature piezoelectric applications, its elevated ferroelectric phase transition (530°C), linear piezoelectric properties under low field and relatively low room temperature conductivity (compared to others Bismuth Titanates) make it very attractive for precision sensors. However, under severe conditions (low frequency, high field, high temperature or low oxygen partial pressure) some of those advantages disappear. Piezoelectric response is dominated by charge drift in general becomes unstable. Above all, at high temperature and low oxygen partial pressure, a large conductivity increase reduces the piezoelectric efficiency of the material, in this work, electrical conductivity, piezoelectric properties and dielectric permittivity of SrBIT ceramic have been investigated in conditions of high temperature, low oxygen partial pressure, low to moderate driving field and frequency. As a result of this research, a better understanding of SrBIT properties was achieved. Thanks to a careful study of SrBIT processing as a bulk ceramic, a reproducible route was established. Many basic mechanisms leading to both SrBIT crystallization and sintering have been identified. It was demonstrated that a detailed knowledge of the exact processing conditions is required in order to achieve high quality material. DC conductivity measurements were carried out as a function of temperature, oxygen partial pressure and dopant concentration. It was found that the apparent activation energy for conduction for undoped SrBIT was 1 eV between 140 and 220°C and 1.5 eV between 450 and 700°C. Decrease of the activation energy in the lower temperature range has been discussed considering grain boundary conductivity, as a transition from electronic to ionic conduction or as consequence of small polarons conduction. It was shown that lower activation energy resulted from Manganese doping (0.5 eV), this was interpreted as either growing influence of grain boundary conductivity as dopant concentration increases or as shallow hole trap formation. DC conductivity measurements and acceptor/donor doping experiments demonstrated p-type conductivity in the low temperature range (up to 220°C) as donor doping decreases conductivity, while oxygen partial pressure controlled measurements indicated n-type conductivity at higher temperature (above 700°C). An electrical impedance analysis was performed with several equivalent circuits. The aim of these models was to simulate the impedance of SrBIT. The best approximation was found with a distributed element of Havriliak-Negami type. However, as the physical justification for this circuit was not clear, the investigation of the grain, grain boundary and electrode impedances was performed with multiple discrete parallel RC elements. With temperature, grain size and oxygen activity variations, the identification of three separate contributions as grain, grain boundary and electrode was realized. The anisotropy of conductivity and permittivity was demonstrated with textured material and both DC and AC analysis. With the master curves built for the electrical modulus, it was found that the impedance probably related to Bismuth oxide layers produces an additional high frequency are. From this finding and by comparing characteristic relaxation frequencies of undoped, 2 mol.% Mn and 4 mol.% Nb SrBIT, it was determined that conductivity is higher in the ab plane direction than in the c direction within both perovskite units and Bismuth oxide layers. With conductivity measurements under controlled oxygen partial pressure, it was found that an acceptor-based (intrinsic or extrinsic) model could be used to describe the electrical conductivity under controlled oxygen partial pressure of both undoped and 2 mol.% Mn doped SrBIT. However, as neither a pO2-1/6 region (intrinsic oxygen vacancies compensated by electrons) for undoped SrBIT nor a pO2-1/4 region (oxygen vacancies compensated by singly ionized acceptors) for Mn doped SrBIT were seen, it was concluded that the acceptorcontrolled model is not sufficient for a complete description of SrBIT. For this reason and in order to include the low oxygen partial pressure behavior of undoped SrBIT, a donor-based (intrinsic) model was also considered. The source of intrinsic donors would be in that case Bi3+ cations sitting on Sr2+ sites in the perovskite sublattice. Considering Bismuth vacancies as the negative compensating species, both pO2-1/4 and pO2-independent regions could be predicted with the model. However, even if the donor-controlled model seems to better match conductivity measurements in the full PO2) range, rejecting the acceptor-based model would be an error. It is actually not demonstrated that in SrBIT the concentration of exchanged Bismuth cations is always (all temperature, pressure) larger than the natural acceptor impurity concentration. It is very likely that the cation exchange is dependent of the oxygen partial pressure. It is also not proved as suggested in the literature that direct compensation between exchanged Strontium and Bismuth exists, reducing the net donor-excess. With the acceptor-controlled model, the mass-action constants for reduction and for ionization of intrinsic carriers across the band gap were determined. The band gap of SrBIT was estimated to be 3.5 eV. The ionic conductivity of SrBIT was determined at high temperature with measurements under controlled oxygen partial pressure. It was found that the electrical conductivity of SrBIT is probably mixed (electronic and ionic) as the estimation of the transference number provided quite large values (t=0.8 at 800°C). From electronic and ionic conductivity data, mixed conduction can actually be predicted in a large temperature range (above room temperature). The piezoelectric measurements using direct effect demonstrated that it is actually possible to unlock piezoelectrically active ferroelectric domain walls and create non-linear piezoelectric properties in undoped SrBIT. This occurs above a threshold elastic field, which is thermally activated. With a piezoelectric composite, it was demonstrated that the electromechanical coupling between two different phases creates a piezoelectric relaxation. This one could be positive or negative depending on the respective properties of each composite's component. It was shown experimentally that a small temperature change is sufficient to transform a positive relaxation into a negative one. While these experiments did not provide a detailed microstructural explanation for the piezoelectric relaxation observed in 2 mol.% Mn doped SrBIT, they gave a first insight into an original phenomenological approach. Microstructure and piezoelectric properties were related with the calculation of a piezoelectric relaxation composite made of two textured samples. This demonstrated that a piezoelectric relaxation may occur just because of anisotropy. Microstructural reproduction of this coupling is actually an important feature of Aurivillius phases.

EPFL2000